1. Field of the Invention
The present invention relates to a linear actuator adapted to be used, for example for an accurate positioning, and particularly to a linear actuator wherein the deformations of a plurality of electrostrictive elements are output as a linear movement of an output shaft.
2. Prior Art
As an example of an above-mentioned linear actuator, an actuator is disclosed in Japanese Provisional patent publication No. 59 (1984)-99977 , as depicted in FIG. 11 of the present application.
As shown in FIG. 11 the linear actuator comprises ring-shaped elements 42, 42 which are composed of two pieces of electrostrictive elements 41, 41 affixed together and fixedly provided at a desired interval within a casing body 43. A cylindrical clamp element 44 extends within the interior circumference of the respective ring-shaped element 42 and is composed of an electrostrictive element which is radially extensible and contractible. An output shaft 45 extends through the clamp elements 44 so as to be moved axially reciprocatively by controlling voltages applied to the ring-shaped elements 42 and the clamp elements 44.
This kind of linear actuator is simple in construction, can be small-sized and is able to attain an ultradecelerated driving force without any reduction gears. Further more, since it has not a magnetic coil such as a solenoid, it is useful as a linear actuator of an ultraaccurate positioning mechanism and the like which refuse to be affected by a magnetic field.
However, since the above-mentioned prior linear actuator is constructed so as to move the output shaft reciprocatively by repeating intermittent movements of the clamp elements 44 (on a microscopic scale thereof), a smooth output can be hardly attained thereby.
Accordingly, to attain a smooth output, multiple clamp elements must be disposed along the output shaft. This is disadvantageous from a manufacturing standpoint as the construction thereof is complicated and both the setup and the adjustment between the clamp elements and the output shaft are difficult.